Any discussion of the background art throughout the specification should in no way be considered as an admission that such art is widely known or forms part of common general knowledge in the field.
Liquid crystal on silicon (LCOS) devices are known in the art for use as optical phase modulators, among other applications. LCOS devices can spatially manipulate optical signals by applying a spatially dependent phase profile to the signals. This has many applications, including beam steering, spectral shaping and signal compensation.
Referring to FIG. 1, there is illustrated schematically a conventional LCOS device 1 including a liquid crystal material 3 sandwiched between a transparent glass layer 5 having a transparent electrode, and a mirror 7 mounted on a silicon substrate 9. The mirror is divided into a two-dimensional array of individually addressable pixels. Each pixel is individually drivable by a voltage signal to provide a local phase change to an optical signal, thereby providing a two-dimensional array of phase manipulating regions. Pre-alignment of the liquid crystal elements within material 3 is provided by alignment layers 11 and 13. These layers generally include a plurality of small grooves induced by rubbing of the polyamide layers or other commonly employed techniques, which align the long axis of the individual liquid crystals to define the slow axis of the liquid crystal element.
As shown in FIG. 1, a liquid crystal material includes elongate molecules that lack positional order but have a large scale orientational order. Due to the elongated shape and ordered orientation of liquid crystals, a liquid crystal medium behaves as an anisotropic dielectric, having an axis of birefringence in the direction of the long axis of the molecules (vertically in FIG. 1). Therefore, liquid crystal based devices, such as LCOS devices, are inherently polarization dependent. Consequently, light transmitted onto a conventional LCOS device at an angle to the molecules' long axis will have one linear polarization component modified in phase to a greater degree than the orthogonal component.
One way to account for this polarization dependence is to spatially separate orthogonal polarization components and send one component through a half wave plate, or two passes through a quarter-wave plate. This results in two separate beams propagating through the device, both of which are of the same polarization orientation. This technique presents practical difficulties in optical devices, particularly in ensuring the optical paths and trajectories of each separated polarization state are identical.
Removal or compensation of the polarization dependence of liquid crystals would reduce or remove the need for implementing polarization diversity into the optical device. This would reduce the number of components required in the system, and therefore potentially reduce the cost and complexity of that system. Several techniques for rendering a liquid crystal polarization independent are outlined below.
In G. D. Love, “Liquid-crystal phase modulator for unpolarized light”, Applied Optics 32, 2222-2223 (1993), it is suggested that a standard nematic liquid crystal LCOS could be made to be polarization insensitive through the combination of a double pass of the LCOS and a quarter wave plate (QWP). This technique is schematically illustrated in FIG. 2, for which reference is now made. As shown, a vertical polarization incident on a liquid crystal cell 15 will be converted into circularly polarized light after propagation through a QWP 17. After one reflection from a mirror 19, the handedness of the polarization switches, and after passing again through the QWP 17, the light passes back through the liquid crystal cell 15 polarized at 90° to the input polarization (corresponding to a 180° phase shift between orthogonal polarization components). In a similar way, a horizontal polarization at the input will emerge vertically polarized. So a given linear input polarization will propagate through the liquid crystal cell 15 as both the original polarization state and also an orthogonal polarization state.
Due to the polarization dependence of the liquid crystal cell 13, the beam will experience a phase change only on one of the two passes through the cell, depending on the polarization state. Importantly, with appropriate orientation of the elements, any input polarization will experience a phase change from the cell.
Earlier, similar techniques for addressing polarization effects were attempted in relation to intensity modulation of liquid crystal display devices (see H. S. Cole and R. A. Kashnow, “New reflective dichroic liquid-crystal display device”, Appl. Phys. Lett. 30, 619-621 (1977)).
While the above described technique is relatively simple to implement, it is considerably inefficient in practice. As the QWP 17 and mirror 19 are inserted adjacent the liquid crystal cell 15, there is a larger distance between the driving electrode (generally located outside mirror 19 or within mirror 19 itself) and the liquid crystal material. Typical liquid crystal cells have a thickness of about 6 to 10 μm. Typical quartz quarter-wave plates have a thickness of about 44 μm for wavelengths of about 1550 nm. This increased thickness requires the application of higher voltages to effectively drive the liquid crystals. In practice, this technique would likely require a high quality QWP that is very thin. Thinner QWP devices based on polymers are currently available. However, these elements still necessarily increase the device thickness, thereby increasing the required drive voltages.
Another technique for achieving polarization independent liquid crystal operation, at least in a transmission configuration, includes implementing twisted nematic liquid crystal material. Twisted nematic liquid crystals twist in proportion to an applied voltage up to an angle of about 90°. This allows modifying the polarization of the liquid crystal by applying a predetermined drive voltage. In J. Patel and S.-D. Lee, “Electrically tunable and polarization insensitive fabry-perot etalon with a liquid-crystal film”, Applied Physics Letters 58, 2491 {2493 (1991), it was shown that a twisted nematic liquid crystal could be used as a tunable Fabry-Perot cavity, and that the resonance was not dependent on polarization. This implies that the change in refractive index was the same for each polarization. FIG. 3 illustrates Fabry-Perot resonance in a twisted nematic liquid crystal cell for unpolarized light, as presented in Patel and Lee. This figure illustrates that there is a threshold for this polarization independence. As shown, at low voltages, the peaks are separated, but at large voltages they follow one another closely. Therefore, at low drive voltages, orthogonally polarized inputs experience a different cavity resonance position. That is, at voltages below the threshold, the liquid crystal is birefringent and polarization dependent. However, at voltages much higher than threshold, the polarization dependence is substantially reduced.
The technique described in Patel and Lee is advantageous in that it can be applied in existing CMOS/liquid crystal contact arrangements. Further, liquid crystals based on twisted nematic molecules are a mature technology and well understood. However, twisted nematic liquid crystals rotate the orientation of a given polarization state. This has implications in many optical systems where the polarization orientation must be strictly controlled. In addition, polarization independence is only achieved at drive voltages greater than the threshold value. Therefore, significant drive voltages may be required.
A third technique for addressing the polarization dependence of liquid crystals is to combine two liquid crystal cells of orthogonal orientation. In such a system, one liquid crystal cell acts on one polarization component and the other cell acts on the orthogonal polarization component. A number of techniques have been studied for implementing such dual-cell systems. Early techniques used a glass spacer between the two liquid crystal cells and attempted to independently address each cell. This technique is practically difficult to achieve using a typical CMOS platform. Further, the presence of the additional spacer increases the device thickness and therefore increases the required voltage to drive the liquid crystals.
Lin et. al, (“Polarization-independent liquid crystal phase modulator using a thin polymer-separated double-layered structure”, Opt. Express 13, 8746-8752 (2005)) discloses a dual liquid crystal cell device having a thin polymer film spacer, allowing the two layers to be treated as a single cell and hence addressed simultaneously. Wu et. Al (“Axially-symmetric sheared polymer network liquid crystals”, Opt. Express 13, 4638-4644 (2005)) discloses dual liquid crystal cells using liquid crystal gels sandwiched together. FIG. 4 schematically illustrates such an arrangement. The gel structure of the liquid crystal materials possesses enough rigidity to stop the two LC layers from mixing together, thereby removing the need for a spacer.
While these dual-cell techniques can provide near perfect polarization independence, they require the development of polymers and new cell arrangements. Furthermore, using sandwiched liquid crystal gels requires high voltages to achieve relatively small phase shifts.
Work has also been undertaken to produce axially symmetric liquid crystals. Theoretically, the axial symmetry makes these cells completely polarization independent. However, this technology is relatively immature and it is likely that a relatively thick cell is required to achieve the required phase change making the cell response slow and transitions between pixels become blurred due to fringing fields. Further, it is not yet clear whether the axial symmetry can be achieved uniformly for each pixel in an LCOS cell.
Y. Pang and R. Gordon “Metal nano-grid reflective wave plate”, Opt. Express 17, 2871-2879 (2009) and A. Vengurlekar, “Polarization dependence of optical properties of metallodielectric gratings with sub-wavelength grooves in classical and conical mounts,” J. Appl. Phys. 104, 023,109-1-023,109-8 (2008) disclose a wave plate formed from a periodic grid structure comprised of alternating metal and dielectric regions. Pang and Gordon suggests that such a grid structure can achieve polarization independent attenuation in a dichroic LC cell or for polarization modulated vertical cavity surface emitting lasers. For applications requiring holographic projection such as are found in display and telecommunications, it can be advantageous to be able to control the output polarization and phase of the output light sequentially in either time or space. For such practical optical applications, for example, stereoscopic projection where the image for each eye is projected holographically in an orthogonal polarization state, more advanced control of phase and polarization is required.
The above techniques for providing polarization independent liquid crystal operation each have their relative disadvantages or shortfalls.